If you want an anal orgasm, you gotta earn it. And probably, you got to train your ass first.
Ride your medium dildo anally, in a position of your choice. Complete 100 nearly full out full in anal thrusts.
Shortly after, put 2 clothing pegs on your ball sack, and complete another 100 nearly full out full in anal thrusts.
Let's see how that goes.
Which chastity device are you using?